Who is the mysterious and playful Deep Throat who is feeding a French judge with tantalising scraps of information on the alleged illegal slush- funding of the President and Prime Minister's political party?

How does the deep throat - known in French as a "corbeau", or crow - get his information, including internal memos from one of France's shadowy security services, the Renseignements Generaux (RG)?

Is the "corbeau" trying to destabilise the RG? The government? Or France itself? Is he, as the head of the RG suggested yesterday, working for the forces of the far-right?

Whoever it is, and whatever his motives, he has a deep knowledge of government procedures and the details of the two-year-old investigation into the byzantine finances of the neo-gaullist RPR, the party founded by President Jacques Chirac and led by the Prime Minister, Alain Juppe. He, or she, also has a cinematic sense of drama and a wicked sense of humour. On one occasion, the "corbeau" left a letter under a windscreen-wiper on the judge's car parked on a Parisian street. It contained a copy of an RG memo on the RPR's finances and a note to the judge, Eric Halphen, asking: "So? Are you still stuck on square one?"

The head of the RG, which combines some of the functions of MI5, the Special Branch and the Jockey Club, took the unusual step yesterday of giving a self-justifying interview on the affair to the newspaper, Le Parisien. It has been suggested that Yves Bertrand, central director of the RG, might be placed formally under investigation for withholding information from Judge Halphen. Mr Bertrand yesterday denied any wrong-doing. He suggested that the RG memos, including one containing words in handwriting remarkably like his own, might be sophisticated forgeries. He suggested that he, the RG, and the Juppe government, were the targets of a politically- motivated plot. He suggested that an unnamed political party, which had called for the abolition of the Renseignements Generaux, might be to blame.

The far-right Front National has called for the scrapping of the RG. Mr Bertrand confirmed to Le Monde yesterday that it was the Front National that he had in mind.

The whole tangled and murky affair goes back to allegations, dating from 1985, that the RPR is funded partly through bank accounts in Switzerland, under the code name Cleo, fed by French companies in kick-backs for lucrative public contracts. The "corbeau" alerted other magistrates, and later Eric Halphen, to the fact that a senior Renseignements Generaux officer, Brigitte Henri, had conducted her own investigation into the finances of the RPR.

Ms Henri denied that she had any useful information and was then, mysteriously, posted to a non-job at the French representation to the EU in Brussels.

Over a period of several months, the "corbeau" sent Judge Halphen several so-called "blancs" - RG memos which are neither titled or signed. The memos, apparently written by Ms Henri in the period 1993-5, contained tantalising references to RPR financing and Swiss banks. One of the memos had the words "urgent et signale" (urgent and notified) scrawled on them, apparently in the handwriting of the RG boss, Mr Bertrand.

Then, last month, the judge received a package from the "corbeau", described as a "New Year's present for 1997". It contained a visiting card belonging to the RG boss with words written - once again in what appeared to be his authentic handwriting - referring, cryptically, to the Cleo bank accounts, Ms Henri and an RPR figure under investigation.

In his interview with Le Parisien yesterday, Mr Bertrand suggested that this was all part of a very elaborate forgery. Neither he nor Ms Henri had ever had any information on RPR financing which could have helped the judge. He had agreed to have his handwriting tested and compared to the examples sent by the "corbeau".

The whole affair remains bizarre - and potentially explosive. None of the information sent by the mystery informant has - in itself - helped Judge Halphen very much. In his most recent dispatch to the judge, the "corbeau" teasingly draws attention to this fact. "Why not the heart of the matter, you might ask. Does he have the heart of the matter? It is time for the crows to go and look for more grain in the swampy fields of politics."